Guided color transient improvement filtering in video coding

ABSTRACT

An encoder receives a target image in a standard dynamic range and a guide image in a high dynamic range, wherein both the target image and the guide image represent the same scene. A color transient improvement (CTI) filter is selected to predict a chroma component of a decoded version of the target image based on both the luma and chroma components of the target image and the guide image. The filtering coefficients for the CTI filter are computed by minimizing an error measurement (e.g., MSE) between pixel values of the decoded image and the guide image. The computed set of filtering coefficients is signaled to a receiver (e.g., as metadata). A decoder receives the coded image and the metadata, and applies the same CTI filter to the decoded image to generate an output image.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/569,125, filed on Aug. 7, 2012, which claims priority to U.S.Provisional Patent Application No. 61/521,685 filed Aug. 9, 2011 andU.S. Provisional Patent Application No. 61/653,234 filed May 30, 2012,each of which is hereby incorporated by reference in its entirety forall purposes.

TECHNOLOGY

The present invention relates generally to images. More particularly, anembodiment of the present invention relates to guided chroma filteringof digital images in video coding.

BACKGROUND

As used herein, the term ‘dynamic range’ (DR) may relate to a capabilityof the human psychovisual system (HVS) to perceive a range of intensity(e.g., luminance, luma) in an image, e.g., from darkest darks tobrightest brights. In this sense, DR relates to a ‘scene-referred’intensity. DR may also relate to the ability of a display device toadequately or approximately render an intensity range of a particularbreadth. In this sense, DR relates to a ‘display-referred’ intensity.Unless a particular sense is explicitly specified to have particularsignificance at any point in the description herein, it should beinferred that the term may be used in either sense, e.g.interchangeably.

As used herein, the term high dynamic range (HDR) relates to a DRbreadth that spans the some 14-15 orders of magnitude of the humanvisual system (HVS). For example, well adapted humans with essentiallynormal vision (e.g., in one or more of a statistical, biometric orophthalmological sense) have an intensity range that spans about 15orders of magnitude. Adapted humans may perceive dim light sources of asfew as a mere handful of photons. Yet, these same humans may perceivethe near painfully brilliant intensity of the noonday sun in desert, seaor snow (or even glance into the sun, however briefly to preventdamage). This span though is available to ‘adapted’ humans, e.g., thosewhose HVS has a time period in which to reset and adjust.

In contrast, the DR over which a human may simultaneously perceive anextensive breadth in intensity range may be somewhat truncated, inrelation to HDR. As used herein, the terms ‘visual dynamic range’ or‘variable dynamic range’ (VDR) may individually or interchangeablyrelate to the DR that is simultaneously perceivable by a HVS. As usedherein, VDR may relate to a DR that spans 5-6 orders of magnitude. Thuswhile perhaps somewhat narrower in relation to true scene referred HDR,VDR nonetheless represents a wide DR breadth. As used herein, the term‘simultaneous dynamic range’ may relate to VDR.

Until fairly recently, displays have had a significantly narrower DRthan HDR or VDR. Television (TV) and computer monitor apparatus that usetypical cathode ray tube (CRT), liquid crystal display (LCD) withconstant fluorescent white back lighting or plasma screen technology maybe constrained in their DR rendering capability to approximately threeorders of magnitude. Such conventional displays thus typify a lowdynamic range (LDR), also referred to as a standard dynamic range (SDR),in relation to VDR and HDR.

As with the scalable video coding and HDTV technologies, extending imageDR typically involves a bifurcate approach. For example, scene referredHDR content that is captured with a modern HDR capable camera may beused to generate either a VDR version or an SDR version of the content,which may be displayed on either a VDR display or a conventional SDRdisplay. In one approach, generating the SDR version from the capturedVDR version may involve applying a global tone mapping operator (TMO) tointensity (e.g., luminance, luma) related pixel values in the HDRcontent. In a second approach, as described in PCT ApplicationPCT/US2011/048861, filed Aug. 23, 2011, “Extending Image Dynamic Range”,by W. Gish et al., generating an SDR image may involve applying aninvertible operator (or predictor) on the VDR data. To conservebandwidth or for other considerations, simultaneous transmission of boththe actual captured VDR content and the SDR content may not be a bestapproach.

Thus, an inverse tone mapping operator (iTMO), inverted in relation tothe original TMO, or an inverse operator in relation to the originalpredictor, may be applied to the SDR content version that was generated,which allows a version of the original VDR content to be predicted. Thepredicted VDR content version may be compared to generated VDR content.For example, subtracting the predicted VDR version from the original VDRversion may generate a residual image. An encoder may send the generatedSDR content as a base layer (BL), any residual image as an enhancementlayer (EL), and package the iTMO or other predictors or as metadata.

Sending the EL and metadata, with its SDR content, residual andpredictors, in a bitstream typically consumes less bandwidth than wouldbe consumed in sending both the HDR and SDR contents directly into thebitstream. Compatible decoders that receive the bitstream sent by theencoder may decode and render the SDR on conventional displays.Compatible decoders however may also use the residual image, the iTMOpredictors, or the metadata to compute a predicted version of the HDRcontent therefrom, for use on more capable displays.

In such layered VDR coding, images may be represented at differentspatial resolutions, bit depths, color spaces, and chroma subsamplingformats, all of which may force a variety of computer-intensivetransformations from a first color format to a second color format.

As used herein, the term “color format” relates to a colorrepresentation that comprises two variables: a) a color space variable(for example: RGB, YUV, YCbCr, and the like) and a chroma subsamplingvariable (for example: 4:4:4, 4:2:0, and the like.) For example, a VDRsignal may have an RGB 4:4:4 color format, while an SDR signal may havea YCbCr 4:2:0 color format.

As used herein, the terms “up-sampling” or “up-scaling” relate to theprocess of transforming one or more color components of an image fromone spatial resolution to a second, higher, spatial resolution. Forexample, a video signal may be up-sampled from a 4:2:0 format to a 4:4:4format.

The approaches described in this section are approaches that could bepursued, but not necessarily approaches that have been previouslyconceived or pursued. Therefore, unless otherwise indicated, it shouldnot be assumed that any of the approaches described in this sectionqualify as prior art merely by virtue of their inclusion in thissection. Similarly, issues identified with respect to one or moreapproaches should not assume to have been recognized in any prior art onthe basis of this section, unless otherwise indicated.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention is illustrated by way of example,and not in way by limitation, in the figures of the accompanyingdrawings and in which like reference numerals refer to similar elementsand in which:

FIG. 1 depicts an example data flow for a layered-coding systemaccording to an embodiment of the present invention;

FIG. 2 depicts an example layered decoding system according to anembodiment of the present invention;

FIG. 3 depicts an example of guided image up-sampling in coding theresidual signal in a layered codec according to an embodiment of thepresent invention;

FIG. 4 depicts an example single-layer video coding system according toan embodiment of the present invention;

FIG. 5 depicts example input and output pixel arrays for up-sampling bya factor of two using a 2D filter according to an embodiment of thepresent invention;

FIG. 6 depicts example input and output pixel arrays for up-sampling bya factor of two using a 3D filter according to an embodiment of thepresent invention;

FIG. 7 depicts an example process for guided image up-sampling accordingto an embodiment of the present invention;

FIG. 8 depicts an example process for guided color transient improvementfiltering according to an embodiment of the present invention.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Guided image up-sampling and color transient improvement filtering invideo coding is described herein. In the following description, for thepurposes of explanation, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, that the present invention may be practicedwithout these specific details. In other instances, well-knownstructures and devices are not described in exhaustive detail, in orderto avoid unnecessarily occluding, obscuring, or obfuscating the presentinvention.

Overview

Example embodiments described herein relate to the guided imageup-sampling and CTI filtering in video coding. An encoder receives afirst image of a first spatial resolution and a guide image of a secondspatial resolution, wherein both the first image and the guide imagerepresent the same scene and the second spatial resolution is higherthan the first spatial resolution. A filter is selected to up-sample thefirst image to a third image with a spatial resolution same as thesecond spatial resolution. The filtering coefficients for theup-sampling filter are computed by minimizing an error measurementbetween pixel values of the guide image and the third image, e.g., themean square error (MSE). The computed set of filtering coefficients issignaled to a receiver (e.g., as metadata). A decoder receives themetadata and the first image or an approximation of the first image andmay up-sample the received image using the same optimized filter andfiltering coefficients as those derived by the encoder.

In another embodiment, an encoder receives a target image to be encoded,the target image comprising a first target color component image and asecond target color component image. The image is encoded and decoded togenerate a coded image and a decoded image, the decoded image comprisinga decoded first color component image and a decoded second colorcomponent image. A color transient improvement (CTI) filter is selectedto filter pixels of the decoded second color component image to generatean output color component image. The CTI filtering coefficients for theCTI filter are computed based, at least in part, by minimizing an errormeasurement between pixel values of the output color component image andcorresponding pixel values of the second color component image in thetarget image. The CTI filtering coefficients are signaled to a receiver(e.g., as metadata). A decoder receives the metadata and the codedimage. After decoding the coded image it may filter the decoded imageusing the same CTI filtering coefficients as those derived by theencoder.

Example Systems with Guided Image Up-Sampling

Image down-sampling and up-sampling transformations play a key role invideo coding since they affect not only coding efficiency but alsooverall image quality. Improper spatial-scaling transformations may leadto false colors, especially along the edges of an image. Unliketraditional “blind” image up-sampling techniques, wherein given an inputimage with a sub-sampled color component (e.g., the chroma in YCbCr4:2:0 format) the color component is up-sampled using only availableinformation within the image itself, embodiments of this invention mayutilize information from other images in the video processing pipelineas well.

FIG. 1 depicts an example image processing system 100 implementingguided image up-sampling according to an embodiment of this invention.System 100 represents an embodiment of a layered encoder, wherein inputsignal V 105 is coded using two layers: a base layer 135 and anenhancement or residual layer 175.

In an embodiment, input signal V 105 may represent an input VDR signalrepresented by 16 or more bits per color component in an 4:4:4 colorformat, e.g., RGB 4:4:4. This VDR signal may be processed by a dynamicrange reduction process 110 to generate signal S′ 112. Signal S′ may bein the same or lower spatial resolution than signal V. Signal S′ may berepresented in a lower bit-depth resolution than V, e.g., 12 bits percolor component. Signal S′ may be in the same color format as V, or inother embodiments, it may be in a different color format.

In an embodiment, coding 130 may be preceded by color transformation120, wherein S′ may be transformed to another color format, e.g., YCbCr.Transformation 120 may also incorporate sub-sampling of one or morecolor components, e.g., from 4:4:4 to 4:2:0. Coded signal 135 may betransmitted as a base layer. In an embodiment, coding 130 may beimplemented by any of the existing video encoders, such as an MPEG-2 orMPEG-4 video encoder, as specified by the motion pictures expert group(MPEG) specifications.

An enhancement layer 175 may be generated by decoding signal 135,generating a predicted value of the original VDR signal V (165), andsubtracting the original V (105) from its predicted value (165) togenerate a residual signal 175. In an embodiment, predictor 160 may beimplemented using multivariate multiple-regression models as describedin PCT application PCT/US2012/033605, filed Apr. 13, 2012, by G-M Su etal. Since the coded signal 135 and the input signal 105 are in differentcolor formats and resolutions, color transform process 150 transformsthe output of the decoder 140 into a color format and resolution thatmatches the color format and resolution of input V 105. For example,unit 150 may transform input data 143 from YCbCr 4:2:0 to RGB 4:4:4.

In traditional up-sampling methods, up-sampled output 153 may be derivedsolely based on sub-sampled input 143 and an interpolation filter withfixed filtering coefficients. In contrast, in an embodiment, theup-sampling process in 150 may perform up-sampling utilizing data fromboth the sub-sampled input 143 and data with known full spatialresolution, e.g., inputs S′112 or V 105. In an embodiment, this guidedimage up-sampling processing 150 may signal up-sampling relatedparameters, such as interpolation filtering coefficients, to the rest ofthe system pipeline (e.g., as metadata signals 155 or 167).

As defined herein, the term “metadata” may relate to any auxiliaryinformation that is transmitted as part of the coded bitstream andassists a decoder to render a decoded image. Such metadata may include,but are not limited to, information as: color space or gamutinformation, dynamic range information, tone mapping information, orother predictor, up-scaling, and quantizer operators, such as thosedescribed herein.

In an embodiment, the filter coefficients used in converting YCbCr 4:2:0data to YCbCr 4:4:4 data are estimated by minimizing an error metric(e.g., the mean square error (MSE)) between the predicted up-sampledvalues (e.g., the output 153 of the color transformation 150) and aninput guide image at full spatial resolution (e.g., S′ 112 or V 105).The same filters may also be applied during the up-sampling process inthe decoder.

FIG. 2 depicts an example implementation of a layered video decoderaccording to an embodiment. Decoding system 200 receives a codedbit-stream 202 that comprises a base layer 227, an enhancement layer (orresidual) 222, and metadata 225, which are extracted followingdemultiplexing 220. For example, in a VDR-SDR system, the base layer 227may represent the SDR representation of the coded signal and themetadata 225 may include information related to the prediction (160) andcolor transform operations (150) used in the encoder. Encoded base layer227 may be decoded using base layer decoder 210 to output decoder SDRsignal S 212. Encoded residual 222 may be decoded (240), de-quantized(250), and added to the output 295 of the predictor 290 to generate theoutput VDR signal V 270.

Color transform unit 230 may incorporate up-sampling (e.g., from 4:2:0to 4:4:4). The up-sampling processor in 230, instead of using “blind”up-sampling techniques that rely only on input data 212, may alsoextract and apply the up-sampling-related parameters (e.g.,interpolation filter coefficients) signaled by an encoder using metadata225 (or 155). Such guided up-sampling during decoding may yield videosignals with improved visual quality at no extra computational cost.

Guided image up-sampling techniques may be applied to other processingsteps in a video coding pipeline as well. FIG. 3 depicts acoding-decoding system of the residual layer in layered codingimplemented according to an embodiment of this invention. Residualsignal R 305 (175) may be in RGB 4:4:4 color format. Residual encoding330 (e.g., using an MPEG-4 video encoder), which typically operates inthe YCbCr 4:2:0 format, is preceded by color transform unit 310, whereininput 305 may be color transformed and down-sampled to YCbCr 4:2:0. Inparallel, up-sampling processor 320 may be guided by input R 305 tocompute optimized up-sampling filtering coefficients according to anembodiment of this invention. These filtering coefficients may besignaled to a decoder, e.g., using metadata 322. On the receiver, colortransform unit 350 may extract the optimized up-sampling filteringcoefficients from metadata 322 and apply them during the process ofup-sampling the decoded YCbCr 4:2:0 data 342 to RGB 4:4:4 data 355.

FIG. 4 depicts an example single-layer video coding system according toan embodiment of the present invention. As depicted in FIG. 4, at leastone color component of input signal V_(I) 405 may be down-sampled byprocessor 410. The down-sampled output 412 is encoded by video encoder430 (e.g., an MPEG-4 encoder) and transmitted to a receiver (400-D)comprising video decoder 440 (e.g., an MPEG-4 decoder) and up-samplingprocessor 450. On the encoder (400-E), up-sampling unit 420 may performguided up-sampling according to methods described in an embodiment ofthis invention and may derive optimized up-sampling filteringcoefficients using information from both the full-resolution input V_(I)405 and the sub-sampled signal 412. The optimized filter coefficientsmay be signaled to the decoder (400-D), e.g., using metadata 422. Hence,the output 442 of the video decoder (430) may be up-sampled inprocessing unit 450 using the same set of optimized coefficients asthose generated by up-sampling unit 420.

Filter Design for Guided Image Up-Sampling

General 2D Non-Separable Filters

For simplicity and without loss of generality, given an input imagecomprising multiple color components (e.g., YCbCr or RGB), we considerthe guided image up-sampling process of a single color component (e.g.,Cb or R). Methods described herein may be repeated as needed for any ofthe image color components requiring up-sampling.

In an example embodiment for up-sampling by a factor of two using a 2Dinterpolation or up-sampling filter, FIG. 5 depicts a 3×3 array of knowninput pixels, denoted as circles 520, to be up-sampled by a factor oftwo and thus generate an array of 6×6 pixels, depicted as squares 510.Denote as c_(j) the input pixels and as ĉ_(j) the predicted orup-sampled pixels, then four new estimated pixels (e.g., 510-22, 510-23,510-32, and 510-33) are generated for each input pixel (e.g., 520-11).In an embodiment, the up-sampling process may be expressed as anon-separable finite impulse response (FIR) filter:

$\begin{matrix}{{{\hat{c}}_{j}^{(i)} = {\sum\limits_{x = 0}^{2}\;{\sum\limits_{y = 0}^{2}\;{m_{xy}^{(i)} \cdot c_{j,{xy}}}}}},{{{for}\mspace{14mu} i} = 0},1,2,3,} & (1)\end{matrix}$where m_(xy) ^((i)) (for x and y from 0 to 2) denote the filteringcoefficients. For example, in FIG. 5, ĉ_(j) ⁽⁰⁾ is denoted as the square510-22.

Consider now a guide image D at the same target resolution as the outputof the up-sampling process, with pixel data denoted herein as d_(j)^((i)). For example, as depicted in FIG. 4, the guide image may be acolor component of the input image V_(I) 405. The problem of derivingthe filter coefficients m_(xy) ^((i)) may be expressed as an errorminimization problem. In an embodiment, filter coefficients aregenerated so that the minimum mean square error (MSE) between the guidepixel data and the predicted up-sampled pixel data is minimized. Thiscan be formulated as:

$\begin{matrix}{{\min\limits_{\{ m_{xy}^{(i)}\}}{\sum\limits_{i = 0}^{3}\;{\sum\limits_{j}^{p - 1}\;{{{\hat{c}}_{j}^{(i)} - d_{j}^{(i)}}}^{2}}}},} & (2)\end{matrix}$where p denotes the total output pixels in the up-sampled image. Forexample, if the output up-sampled image has a resolution of m rows and ncolumns, then for each color component p=mn.

Equation (2) can be solved using a variety of known numericaltechniques, such as those described in “Applied Multivariate StatisticalAnalysis,” by R. A. Johnson, and D. W. Wichern, 5th Edition, PrenticeHall, 2001. In an example embodiment, denote as

$\begin{matrix}{M = \begin{bmatrix}m_{00}^{(0)} & m_{00}^{(1)} & m_{00}^{(2)} & m_{00}^{(3)} \\m_{01}^{(0)} & m_{01}^{(1)} & m_{01}^{(2)} & m_{01}^{(3)} \\m_{02}^{(0)} & m_{02}^{(1)} & m_{02}^{(2)} & m_{02}^{(3)} \\m_{10}^{(0)} & m_{10}^{(1)} & m_{10}^{(2)} & m_{10}^{(3)} \\m_{11}^{(0)} & m_{11}^{(1)} & m_{11}^{(2)} & m_{11}^{(3)} \\m_{12}^{(0)} & m_{12}^{(1)} & m_{12}^{(2)} & m_{12}^{(3)} \\m_{20}^{(0)} & m_{20}^{(1)} & m_{20}^{(2)} & m_{20}^{(3)} \\m_{21}^{(0)} & m_{21}^{(1)} & m_{21}^{(2)} & m_{21}^{(3)} \\m_{22}^{(0)} & m_{22}^{(1)} & m_{22}^{(2)} & m_{22}^{(3)}\end{bmatrix}} & (3)\end{matrix}$the filtering coefficients.

For a 3×3 filter, given input pixel data vector (520)c _(j) =└c _(j,01) c _(j,02) c _(j,10) c _(j,11) c _(j,12) c _(j,20) c_(j,21) c _(j,22)┘,  (4)a p×9 matrix C may be formed from the input sub-sampled data as

$\begin{matrix}{C = {\begin{bmatrix}c_{0} \\c_{1} \\\vdots \\c_{p - 1}\end{bmatrix}.}} & (5)\end{matrix}$

Similarly, a p×4 guiding-data matrix R may be formed as

$\begin{matrix}{{R = \begin{bmatrix}r_{0} \\r_{1} \\\vdots \\r_{p - 1}\end{bmatrix}},} & (6)\end{matrix}$wherer _(j) =[d _(j) ⁽⁰⁾ d _(j) ⁽¹⁾ d _(j) ⁽²⁾ d _(j) ⁽³⁾ ],j=0,1, . . .,p−1.  (7)

Then, the estimated (up-sampled) pixels may be expressed as

$\begin{matrix}{{\hat{R} = \begin{bmatrix}{\hat{r}}_{0} \\{\hat{r}}_{1} \\\vdots \\{\hat{r}}_{p - 1}\end{bmatrix}},} & (8)\end{matrix}$where{circumflex over (r)} _(j) =[ĉ _(j) ⁽⁰⁾ ĉ _(j) ⁽¹⁾ ĉ _(j) ⁽²⁾ ĉ _(j) ⁽³⁾],j=0,1, . . . ,p−1.  (9)

From equations (1-9), the estimated pixels can be expressed as{circumflex over (R)}=CM.  (10)In an embodiment, the optimization goal is to minimize the estimatederror between the guide input data and the estimated output up-sampleddata, which from equation (2), can be expressed as:

$\begin{matrix}{\min\limits_{M}{{{R - \hat{R}}}^{2}.}} & (11)\end{matrix}$A solution, optimum in the least squares sense, may be obtained viaM=(C ^(T) C)⁻¹ C ^(T) R  (12)

From equation (3), for each color component to be up-sampled (e.g., Cband Cr), this embodiment may compute 4*9=36 coefficients which may besignaled to the decoder using metadata or other means. The set of filtercoefficients (M) can be updated as needed. In some embodiments, for agiven color component, M may be computed once for each frame. In otherembodiments, M may be updated on a per image region basis. In some otherembodiments, M may be updated on a per scene basis. In otherembodiments, M values computed for one color component (e.g., Cb) may beapplied to other color components of the same image as well (e.g., Cr).

Regarding the embodiment of FIG. 1, up-sampling in color transform unit150 may use as guide image R pixels from either input image S′ 112 orinput image V 105.

As depicted in FIG. 5 and denoted in equation (1), an embodiment of 1:2pixel up-sampling filtering utilizes a 3×3 2D filter. The methodsdescribed herein can easily be extended to support other filter sizes,e.g., 2×2, 5×5, 7×7, and the like, and other up-sampling ratios (e.g.,1:3, 1:4, and the like). The methods are also applicable to simpler, 1Dfilters. In an embodiment, in equations (1) and (3), a 3-tap 1D filtercan be defined by setting m_(0y) ^((i))=0 and m_(2y) ^((i))=0 andsolving only for m_(1y) ^((i)) (for y=0, 1, and 2).

Symmetric 2D Non-separable Filters

In equation (1), each up-sampled image pixel is predicted using its ownset of filtering coefficients. To reduce the number of coefficients thatneed to be signaled to a decoder, the total number of filtercoefficients may be reduced by sharing filter coefficients acrossup-sampled pixels. In an example embodiment, even and odd rows of theup-sampled image data may be estimated using the same filtercoefficients.

Returning to FIG. 5, in an embodiment, the up-sampling process may beformulated as follows. For each input sample, c_(j), two samples (ĉ_(j)⁽⁰⁾, ĉ_(j) ⁽¹⁾) (510-22 and 510-23) are estimated in an odd row and twosamples (ĉ_(j) ⁽²⁾, ĉ_(j) ⁽³⁾) (510-32 and 510-33) are estimated in aneven row. For each up-sampled pixel, the up-sampling process may beexpressed using two 2D FIR filters that share their filteringcoefficients:

$\begin{matrix}{{{\hat{c}}_{j}^{(i)} = {{{\sum\limits_{y = 0}^{2}\;{m_{0y}^{(i)} \cdot c_{j,{0y}}}} + {\sum\limits_{y = 0}^{2}\;{m_{1y}^{(i)} \cdot c_{j,{1y}}}} + {\sum\limits_{y = 0}^{2}\;{{m_{2y}^{(i)} \cdot c_{j,{2y}}}\mspace{14mu}{for}\mspace{14mu} i}}} = 0}},1,{and}} & (13) \\{{\hat{c}}_{j}^{(i)} = {{\sum\limits_{y = 0}^{2}\;{m_{0y}^{({i - 2})} \cdot c_{j,{2y}}}} + {\sum\limits_{y = 0}^{2}\;{m_{1y}^{({i - 2})} \cdot c_{j,{1y}}}} + {\sum\limits_{y = 0}^{2}\;{{m_{2y}^{({i - 2})} \cdot c_{j,{0y}}}\mspace{14mu}{\quad{{{{for}\mspace{14mu} i} = 2},3.}}}}}} & (14)\end{matrix}$

Similarly to the previous embodiment, let

$\begin{matrix}{{M = \begin{bmatrix}m_{00}^{(0)} & m_{00}^{(1)} \\m_{01}^{(0)} & m_{01}^{(1)} \\m_{02}^{(0)} & m_{02}^{(1)} \\m_{10}^{(0)} & m_{10}^{(1)} \\m_{11}^{(0)} & m_{11}^{(1)} \\m_{12}^{(0)} & m_{12}^{(1)} \\m_{20}^{(0)} & m_{20}^{(1)} \\m_{21}^{(0)} & m_{21}^{(1)} \\m_{22}^{(0)} & m_{22}^{(1)}\end{bmatrix}},} & (15)\end{matrix}$denote a 9×2 matrix of the filter coefficients used in equations (13)and (14).

Let

$\begin{matrix}{{c_{j}^{odd} = \begin{bmatrix}c_{j,00} & c_{j,01} & c_{j,02} & c_{j,10} & c_{j,11} & c_{j,12} & c_{j,20} & c_{j,21} & c_{j,22}\end{bmatrix}},} & (16) \\{\mspace{79mu}{{C^{odd} = \begin{bmatrix}c_{0}^{odd} \\c_{1}^{odd} \\\vdots \\c_{{p/2} - 1}^{odd}\end{bmatrix}},}} & (17) \\{{c_{j}^{even} = \begin{bmatrix}c_{j,20} & c_{j,21} & c_{j,22} & c_{j,10} & c_{j,11} & c_{j,12} & c_{j,00} & c_{j,01} & c_{j,02}\end{bmatrix}},} & (18) \\{\mspace{79mu}{{C^{even} = \begin{bmatrix}c_{0}^{even} \\c_{1}^{even} \\\vdots \\c_{{p/2} - 1}^{even}\end{bmatrix}},}} & (19) \\{\mspace{79mu}{and}} & \; \\{\mspace{79mu}{{C = \begin{bmatrix}C^{odd} \\C^{even}\end{bmatrix}},}} & (20)\end{matrix}$and let input guide data

$\begin{matrix}{{r_{j}^{odd} = \begin{bmatrix}d_{j}^{(0)} & d_{j}^{(1)}\end{bmatrix}},} & (21) \\{{R^{odd} = \begin{bmatrix}r_{0}^{odd} \\r_{1}^{odd} \\\vdots \\r_{{p/2} - 1}^{odd}\end{bmatrix}},} & (22) \\{{r_{j}^{even} = \begin{bmatrix}d_{j}^{(2)} & d_{j}^{(3)}\end{bmatrix}},} & (23) \\{{R^{even} = \begin{bmatrix}r_{0}^{even} \\r_{1}^{even} \\\vdots \\r_{{p/2} - 1}^{even}\end{bmatrix}},} & (24) \\{R = {\begin{bmatrix}R^{odd} \\R^{even}\end{bmatrix}.}} & (25)\end{matrix}$Then, the estimated up-sampled pixels can be expressed as{circumflex over (R)}=CM,  (26)and from equation (11) and equations (16-26) a solution M, optimum inthe least squares sense, may be obtained viaM=(C ^(T) C)⁻¹ C ^(T) R  (27)General 3D Non-separable filter

In certain embodiments, at least one color component (e.g., the Y (Luma)component in YCbCr or the Green (G) component in RGB) may not undergoany down-sampling, thus preserving important edge-related information.The up-sampling process of the remaining color components (e.g., Cb andCr in YCbCr) may be further improved if the 2D up-sampling filtersdescribed earlier are extended to become 3D filters as described herein.

FIG. 6 depicts example input and output pixels arrays for up-sampling bya factor of two by applying a 3D filter according to an embodiment ofthis invention. As before, denote as c_(j) the input pixels of the colorcomponent to be up-sampled (e.g., Cb) and as ĉ_(j) the predicted orup-sampled pixels. Denote as y_(j) (630) pixels of the same guide imagebut from another color component available at full resolution (e.g., Y).In an embodiment, assuming without loss of generality that eachup-sampled pixel is computed using a 3×3 2D filter on the input data(c_(j)) and a 4×2 filter on a separate component of the input guide data(e.g., the y_(j) pixels), four new estimated pixels may be generated foreach input pixel using a 3D filter described by:

$\begin{matrix}{{{\hat{c}}_{j}^{(i)} = {\left( {\sum\limits_{x = 0}^{2}{\sum\limits_{y = 0}^{2}{m_{xy}^{(i)} \cdot c_{j,{xy}}}}} \right) + \left( {\sum\limits_{x = 0}^{3}{\sum\limits_{y = 0}^{1}{n_{xy}^{(i)} \cdot y_{j,{xy}}}}} \right)}},{{{for}\mspace{14mu} i} = 0},1,2,3,} & (28)\end{matrix}$where m_(xy) ^((i)) and n_(xy) ^((i)) denote the filter coefficients.These filter coefficients can be derived as before by solving anoptimization problem

$\begin{matrix}{{\min\limits_{M}{{R - \hat{R}}}^{2}},} & (29)\end{matrix}$where R is a matrix of guiding image data as defined in equation (6) and{circumflex over (R)}=CM,  (30)where given M a 9×4 matrix with the m_(xy) ^((i)) coefficients (similarto matrix denoted in equation (3)) and N a 8×4 matrix with the n_(xy)^((i)) coefficients,

$\begin{matrix}{M = {\begin{bmatrix}\overset{\_}{M} \\\overset{\_}{N}\end{bmatrix}.}} & (31)\end{matrix}$

Matrix C represents the observed pixel data. Given

$\begin{matrix}{{c_{j} = \left\lfloor \begin{matrix}c_{j,00} & c_{j,01} & c_{j,02} & c_{j,10} & c_{j,11} & c_{j,12} & c_{j,20} & c_{j,21} & c_{j,22}\end{matrix} \right\rfloor},} & (32) \\{\mspace{79mu}{{\overset{\_}{C} = \begin{bmatrix}c_{0} \\c_{1} \\\vdots \\c_{p - 1}\end{bmatrix}},}} & (33) \\{\mspace{79mu}{{y_{j} = \left\lfloor \begin{matrix}y_{j,00} & y_{j,01} & y_{j,10} & y_{j,11} & y_{j,20} & y_{j,21} & y_{j,30} & y_{j,31}\end{matrix} \right\rfloor},}} & (34) \\{and} & \; \\{\mspace{79mu}{{\overset{\_}{Y} = \begin{bmatrix}y_{0} \\y_{1} \\\vdots \\y_{p - 1}\end{bmatrix}},}} & (35)\end{matrix}$thenC=[ C Y],  (36)and as before, the optimal solution in the mean square sense may bedefined byM=(C ^(T) C)⁻¹ C ^(T) R  (37)Symmetric 3D Non-separable filter

The methods described earlier can easily be extended to embodiments thatmay apply 2D and 3D filters with different number of pixel coefficientsand different up-sampling ratios.

As described in the 2D-filter case, in equation (28), the number ofcoefficients that need to be signaled to a decoder can be reduced ifeven and odd rows of the up-sampled image data are estimated using thesame filter coefficients. In an embodiment, the prediction equations canbe denoted as

for i=0 and 1

$\begin{matrix}{{\hat{c}}_{j}^{(i)} = {{\sum\limits_{y = 0}^{2}{m_{0y}^{(i)} \cdot c_{j,{0y}}}} + {\sum\limits_{y = 0}^{2}\;{m_{1y}^{(i)} \cdot c_{j,{1y}}}} + {\sum\limits_{y = 0}^{2}\;{m_{2y}^{(i)} \cdot c_{j,{2y}}}} + {\sum\limits_{y = 0}^{1}{n_{0y}^{({i - 2})} \cdot y_{j,{0y}}}} + {\sum\limits_{y = 0}^{1}{n_{1y}^{({i - 2})} \cdot y_{j,{1y}}}} + {\sum\limits_{y = 0}^{1}{n_{2y}^{({i - 2})} \cdot y_{j,{2y}}}} + {\sum\limits_{y = 0}^{1}{n_{3y}^{({i - 2})} \cdot y_{j,{3y}}}}}} & (38)\end{matrix}$for i=2 and 3

$\begin{matrix}{{\hat{c}}_{j}^{(i)} = {{\sum\limits_{y = 0}^{2}{m_{0y}^{({i - 2})} \cdot c_{j,{2y}}}} + {\sum\limits_{y = 0}^{2}{m_{1y}^{({i - 2})} \cdot c_{j,{1y}}}} + {\sum\limits_{y = 0}^{2}{m_{2y}^{({i - 2})} \cdot c_{j,{0y}}}} + {\sum\limits_{y = 0}^{1}{n_{0y}^{({i - 2})} \cdot y_{j,{3y}}}} + {\sum\limits_{y = 0}^{1}{n_{1y}^{({i - 2})} \cdot y_{j,{2y}}}} + {\sum\limits_{y = 0}^{1}{n_{2y}^{({i - 2})} \cdot y_{j,{1y}}}} + {\sum\limits_{y = 0}^{1}{n_{3y}^{({i - 2})} \cdot {y_{j,{0y}}.}}}}} & (39)\end{matrix}$

Using equations (38) and (39) and the same methodology as appliedearlier, optimum filter coefficients m_(xy) ^((i)) and n_(xy) ^((i)) canbe derived.

Example Process for Guided Up-Sampling

FIG. 7 shows an example process for guided image up-sampling accordingto an example implementation of this invention. The process starts atstep 710, wherein an up-sampling processor, such as processors 150, 320,or 420, receives an input image to be up-sampled (e.g., images 143, 312,or 412) and an input guide image (e.g., inputs 112, 305, or 405),wherein the guide image has a spatial resolution higher than the spatialresolution of the input image. Given these two inputs, in step 720 anup-sampling filter is determined (e.g. a general 2D non-separable filteror a symmetric 3D non-separable filter). The up-sampling filter may befixed and known by both the encoder and the decoder, or as describedearlier, the up-sampling process may select a filter among a variety ofup-sampling filters, including (but not necessarily limited to): 1Dup-sampling filters, general 2D up-sampling filters, symmetric 2Dup-sampling filters, general 3D up-sampling filters, or symmetric 3Dup-sampling filters. The selection of the up-sampling filter may beperformed using a variety of methods that may take into considerations anumber of criteria, including available computational and memoryresources, the MSE prediction error using a particular filter, andtarget coding efficiency.

Given the up-sampling filter model, a set of filter coefficients, M, maybe derived, according to a pre-defined optimization criterion. Forexample, under the MSE criterion, optimum solutions for M may be derivedusing MSE optimization techniques as those described herein whichminimize the MSE between the guide image samples and the predictedsamples of the up-sampled image.

After solving for the filter coefficients M, in some embodiments, thecoefficients and (optionally) the identity of the up-sampling filter maybe transmitted to a receiver, e.g., as metadata.

Up-sampling process 700 may be repeated at a variety of intervals asdeemed necessary to maintain coding efficiency while using availablecomputing resources. For example, when coding video signals, process 700can be repeated for each frame, a group of frames, a part of frame, orwhenever the prediction residual between guide images and up-sampledimages exceeds a particular threshold.

Guided Color Transient Improvement (CTI) Filtering

As used herein, the term “color transient artifacts” denotescolor-related artifacts in image or video processing. For example, invideo and image coding one may recognize such artifacts by the presenceof false colors in a decoded image (e.g., across the edges of anobject). Such artifacts may also be referred to as “color bleeding.”Color transient artifacts may occur when each color plane of an image isprocessed separately using different levels of compression. For example,in an RGB image, the Red and Blue planes may be quantized differentlythan the Green plane. Similarly, in a YCbCr image, the Cb and Cr planesmay be processed differently than the Y plane.

One approach to reduce color transients artifacts is to applypost-processing filtering to the chroma or secondary color components ofthe decoded image. Similarly to the guided up-sampling process describedearlier, guided CTI filtering derives optimum filtering coefficients inthe encoder and transmits them as metadata to the decoder. Furthermore,filtering of a pixel in a color plane (e.g., Cb in YCbCr or B in RGB)may take into consideration both neighbor pixels in the same color planeand neighbor pixels of a corresponding pixel in another color plane(e.g., the luminance, Y, plane in YCbCr or G in RGB).

In an embodiment, optimal filter coefficients may be derived in theencoder based on the original, uncompressed, data. The filtercoefficients may be estimated on a per block, frame, or scene basis,depending on available resources and bandwidth. On the decoder, thefilter may be applied as a post-processing (out-of-loop) filter toimprove overall picture quality.

Example Derivation of Filter Coefficients in a CTI Filter

Denote as c_(ij) the input pixels of an image color component (e.g., Cb,Cr, R, or B) to be filtered using a CTI filter, and as ĉ_(ij) the outputfiltered pixels. Denote as y_(ij) pixels of the same image to befiltered but from a second color component (e.g., luma Y or G). In anembodiment, assuming without loss of generality that each color pixel isfiltered using a generalized 3D filter that applies a (2N+1)×(2N+1)kernel for the first color component and a (2M+1)×(2M+1) kernel for thesecond color component (e.g. 3×3 and 3×3 when N=M=1), then the filteredoutput may be denoted as:

$\begin{matrix}{{{\hat{c}}_{ij} = {\left( {\sum\limits_{x = {- N}}^{N}\;{\sum\limits_{y = {- N}}^{N}{m_{xy} \cdot c_{{({i - x})}{({j - y})}}}}} \right) + \left( {\sum\limits_{x = {- M}}^{M}\;{\sum\limits_{y = {- M}}^{M}{n_{xy} \cdot y_{{({i - x})}{({j - y})}}}}} \right)}},} & (40)\end{matrix}$where m_(xy) and n_(xy) denote the filter coefficients.

One may recognize that equation (40) is very similar to the generalized3D up-sampling filter described by equation (28); hence filtercoefficients in equation (40) may be derived as before by solving theoptimization problem

$\begin{matrix}{{\min\limits_{{\{ m_{xy}\}},{\{ n_{xy}\}}}{\sum\limits_{j}^{p - 1}\;{{d_{ij} - {\hat{c}}_{ij}}}^{2}}},} & (41)\end{matrix}$where d_(ij) denotes the pixels of a reference or guide image (e.g.,input V 105).

As before, equation (40) may be expressed in a matrix form as{circumflex over (R)}=CM,  (42)and the solution to equation (41) can be expressed as an optimizationproblem

$\begin{matrix}{{\min\limits_{M}{{R - \hat{R}}}^{2}},} & (43)\end{matrix}$where R represents a vector comprising the guide image data (d_(ij)), Mis a (2N+1)²×1 vector with the m_(xy) coefficients, and N is a (2M+1)²×1vector with the n_(xy) coefficients, so that

$\begin{matrix}{M = {\begin{bmatrix}\overset{\_}{M} \\\overset{\_}{N}\end{bmatrix}.}} & (44)\end{matrix}$

Matrix C represents the observed pixel data (c_(ij) and y_(ij))C=[ C Y].  (45)As described earlier, the optimal solution of equation (43) in the meansquare sense may be expressed asM=(C ^(T) C)⁻¹ C ^(T) R  (46)

The process may be repeated for each of the color components thatrequire CIT filtering (e.g., Cb and Cr or R and B).

FIG. 8 depicts an example process for guided color transient improvementfiltering according to an embodiment of the present invention. In step810, the encoder (e.g., system 100 depicted in FIG. 1) may firstreconstruct an estimate of the decoded image (e.g., V 270) as it will bereceived by the receiver (e.g., system 200 depicted in FIG. 2). Forexample, the reconstructed picture on the decoder (e.g., 270) may beestimated by adding the output of the predictor 160 (e.g., signal 165)to a coded and then decoded version of the residual 175.

In step 820, using equation (46), with R based on input V 105, theencoder may derive the optimum CTI filter coefficients M. In step 830,these coefficients and other filtering parameters may be transmitted tothe decoder as part of the metadata bit stream. On the decoder, afterreconstructing signal V 270, a separate post-processing process mayapply a CTI filter to signal V 270 to improve overall quality byreducing color-related artifacts.

Process 800 may be repeated at a variety of intervals as deemednecessary to maintain coding efficiency while using available computingresources. For example, when coding video signals, process 800 may berepeated for each frame, a group of frames, a part of frame, or wheneverthe prediction residual between guide images and CTI filtered imagesexceeds a particular threshold. Process 800 may also be repeated foreach of the color components that may require CTI filtering.

Example Computer System Implementation

Embodiments of the present invention may be implemented with a computersystem, systems configured in electronic circuitry and components, anintegrated circuit (IC) device such as a microcontroller, a fieldprogrammable gate array (FPGA), or another configurable or programmablelogic device (PLD), a discrete time or digital signal processor (DSP),an application specific IC (ASIC), and/or apparatus that includes one ormore of such systems, devices or components. The computer and/or IC mayperform, control, or execute instructions relating to guided up-samplingor CTI filtering, such as those described herein. The computer and/or ICmay compute any of a variety of parameters or values that relate toguided image up-sampling as described herein. The image and videoembodiments may be implemented in hardware, software, firmware andvarious combinations thereof.

Certain implementations of the invention comprise computer processorswhich execute software instructions which cause the processors toperform a method of the invention. For example, one or more processorsin a display, an encoder, a set top box, a transcoder or the like mayimplement guided image up-sampling or CTI filtering methods as describedabove by executing software instructions in a program memory accessibleto the processors. The invention may also be provided in the form of aprogram product. The program product may comprise any medium whichcarries a set of computer-readable signals comprising instructionswhich, when executed by a data processor, cause the data processor toexecute a method of the invention. Program products according to theinvention may be in any of a wide variety of forms. The program productmay comprise, for example, physical media such as magnetic data storagemedia including floppy diskettes, hard disk drives, optical data storagemedia including CD ROMs, DVDs, electronic data storage media includingROMs, flash RAM, or the like. The computer-readable signals on theprogram product may optionally be compressed or encrypted.

Where a component (e.g. a software module, processor, assembly, device,circuit, etc.) is referred to above, unless otherwise indicated,reference to that component (including a reference to a “means”) shouldbe interpreted as including as equivalents of that component anycomponent which performs the function of the described component (e.g.,that is functionally equivalent), including components which are notstructurally equivalent to the disclosed structure which performs thefunction in the illustrated example embodiments of the invention.

EQUIVALENTS, EXTENSIONS, ALTERNATIVES AND MISCELLANEOUS

Example embodiments that relate to guided image up-sampling and CTIfiltering are thus described. In the foregoing specification,embodiments of the present invention have been described with referenceto numerous specific details that may vary from implementation toimplementation. Thus, the sole and exclusive indicator of what is theinvention, and is intended by the applicants to be the invention, is theset of claims that issue from this application, in the specific form inwhich such claims issue, including any subsequent correction. Anydefinitions expressly set forth herein for terms contained in suchclaims shall govern the meaning of such terms as used in the claims.Hence, no limitation, element, property, feature, advantage or attributethat is not expressly recited in a claim should limit the scope of suchclaim in any way. The specification and drawings are, accordingly, to beregarded in an illustrative rather than a restrictive sense.

What is claimed is:
 1. A method for guided filtering by an encoder, themethod comprising: receiving a target image to be encoded by an encoderand a guide image, wherein both the target image and the guide imagerepresent similar scenes and each comprises a first color component anda second color component; encoding the target image with the encoder togenerate a coded image; decoding the coded image with a decoder togenerate a decoded image, the decoded image comprising a decoded firstcolor component and a decoded second color component; selecting a colortransient improvement (CTI) filter to filter pixels of the decoded imageto generate an output color component image; computing filteringcoefficients for the color transient improvement (CTI) filter, whereinthe filtering coefficient computation is based on minimizing an errormeasurement between pixel values of the output color component image andcorresponding pixel values of the second color component in the guideimage, wherein the CTI filter comprises a first set of filteringcoefficients and a second set of filtering coefficients, whereingenerating the output color component image comprises combining theresult of filtering the first color component of the decoded image withthe first set of filtering coefficients with the result of filtering thesecond color component of the decoded image with the second set offiltering coefficients; and transmitting the CTI filtering coefficientsto a decoder.
 2. The method of claim 1, further comprising combining theCTI filtering coefficients with the coded image in a coded bitstream andtransmitting the coded bitstream to the decoder.
 3. The method of claim1, wherein the CTI filter is selected from among a plurality ofavailable filters.
 4. The method of claim 3, wherein the plurality ofavailable filters comprise a general three dimensional (3D) filter and asymmetric 3D filter.
 5. The method of claim 3, further comprisingsignaling to the decoder an identity of the selected CTI filter.
 6. Themethod of claim 1, wherein the error measurement comprises a mean squareerror (MSE) computation.
 7. The method of claim 1, wherein the firstcolor component of the target image, the guide image, or the decodedimage is a luminance (Y) component and the second color component of thetarget image, the guide image, or the decoded image is a chroma (Cb orCr) color component.
 8. The method of claim 1 wherein the first colorcomponent of the target image, the guide image, or the decoded image isa Green (G) component, and the second color component of the targetimage, the guide image, or the decoded image is a Red (R) or Blue (B)color component.
 9. The method of claim 1, wherein the guide image has ahigher dynamic range than the target image.
 10. The method of claim 9,wherein the guide image has a visual or high dynamic range and thetarget image has a standard dynamic range (SDR).
 11. A non-transitorycomputer-readable storage medium having stored thereoncomputer-executable instruction for executing a method with one or moreprocessors in accordance with the method of claim
 1. 12. A method forguided filtering in a decoder, the method comprising: receiving by thedecoder a coded image and a set of color transient improvement (CTI)filtering coefficients for a CTI filter, wherein the CTI filteringcoefficients comprise a first set of filtering coefficients and a secondset of filtering coefficients; decoding the coded image to generate adecoded image comprising a first color component and a second colorcomponent; and filtering the decoded image with the CTI filter togenerate a filtered color component image, wherein generating thefiltered color component image comprises combining the result offiltering the first color component of the decoded image with the firstset of filtering coefficients with the result of filtering the secondcolor component of the decoded image with the second set of filteringcoefficients, wherein the CTI filtering coefficients are generated by anencoder and are based on: decoding the coded image in the encoder togenerate a decoded image in the encoder; and minimizing an errormeasurement between pixel values of the CTI filter decoded image in theencoder and corresponding pixel values of a guide image received by theencoder, wherein the guide image represents the same scene as thedecoded.
 13. The method of claim 12, wherein the error measurementcomprises a mean square error (MSE) computation.
 14. The method of claim11, further comprising combining the filtered color component image withthe first color component of the decoded image to generate an outputimage.
 15. The method of claim 11, wherein the first color component ofthe decoded image is a luminance (Y) component and the second colorcomponent of the decoded image is a chroma (Cb or Cr) color component.16. An apparatus comprising a processor and configured to perform themethod recited in claim
 12. 17. A non-transitory computer-readablestorage medium having stored thereon computer-executable instruction forexecuting a method with one or more processors in accordance with themethod of claim 12.